The Astrophysical Journal, 809:117 (9pp), 2015 August 20 doi:10.1088/0004-637X/809/2/117 © 2015. The American Astronomical Society. All rights reserved.
A PROBABLE MILLI-PARSEC SUPERMASSIVE BINARY BLACK HOLE IN THE NEAREST QUASAR MRK 231 Chang-Shuo Yan1, Youjun Lu1, Xinyu Dai2, and Qingjuan Yu3 1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, China; [email protected] 2 Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, Norman OK, 73019, USA 3 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, 100871, China Received 2015 May 8; accepted 2015 July 16; published 2015 August 14
ABSTRACT Supermassive binary black holes (BBHs) are unavoidable products of galaxy mergers and are expected to exist in the cores of many quasars. Great effort has been made during the past several decades to search for BBHs among quasars; however, observational evidence for BBHs remains elusive and ambiguous, which is difficult to reconcile with theoretical expectations. In this paper, we show that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as emission from accretion flows onto a BBH, with a semimajor axis of ∼590 AU and an orbital period of ∼1.2 years. The flat optical and UV continua are mainly emitted from a circumbinary disk and a mini- disk around the secondary black hole (BH), respectively; and the observed sharp drop off and flux deficit at λ ∼ 4000–2500 Å is due to a gap (or hole) opened by the secondary BH migrating within the circumbinary disk. If confirmed by future observations, this BBH will provide a unique laboratory to study the interplay between BBHs and accretion flows onto them. Our result also demonstrates a new method to find sub-parsec scale BBHs by searching for deficits in the optical-to-UV continuum among the spectra of quasars. Key words: accretion, accretion disks – black hole physics – galaxies: active – galaxies: individual (Mrk 231) – galaxies: nuclei – quasars: supermassive black holes
1. INTRODUCTION predictions, and the paper is the first attempt to apply this method to fit real observations. Supermassive binary black holes (BBHs) are natural In this paper, we report a BBH candidate in the core of products of the hierarchical mergers of galaxies in the Mrk 231, the nearest quasar with a redshift z = 0.0422, Λ CDM cosmology and are expected to be abundant (e.g., according to its unique optical-UV spectrum. In Section 2,we Begelman et al. 1980;Yu2002; Merritt and Milosavlje- summarize the multi-wavelength spectrum of Mrk 231 and its vić 2005), since many galaxies (if not all) are found to host a distinctive spectral features comparing with normal quasars. supermassive black hole (SMBH) at their centers (e.g., ) The spectrum of Mrk 231 at the optical band is similar to the Magorrian et al. 1998; Kormendy & Ho 2013 . Evidence has quasar composite spectrum; however, it drops dramatically at been accumulated for SMBH pairs in active galactic nuclei the wavelengths around 3000 Å and becomes flat again at (AGNs) and quasars with perturbed galaxy morphologies or 2500 Å. This anomalous continuum is hard to be explained other merger features (e.g., Komossa et al. 2003; Liu / ( ) ) by normal extinction absorption Veilleux et al. 2013 .We et al. 2010; Comerford et al. 2011; Fu et al. 2012 . These propose that the unique optical-to-UV spectrum of Mrk 231 can SMBH pairs will unavoidably evolve to closely bound BBHs be explained by emission from a BBH accretion system, with with separations less than 1 pc. However, the evidence for which the drop of the continuum at 4000Å is due to a gap or BBHs at the sub-parsec scale is still elusive (e.g., Popo- ć ) a hole opened by the secondary component of the BBH. vi 2012 , which raises a challenge to our understanding of the In Section 3, we introduce a simple (triple-)disk model for BBH merger process and the formation and evolution of the accretion onto a BBH system. Using this model, we fit SMBHs and galaxies. the optical-to-UV continuum of Mrk 231 (Section 4) and A number of BBH candidates in quasars have been proposed constrain the orbital configuration of the BBH system and the according to various spectral or other features, such as the ( associated physical parameters of the accretion process in double-peaked, asymmetric, or offset broad line emission e.g., Section 5. Discussions and conclusions are given in Sections 6 Boroson & Lauer 2009; Tsalmantza et al. 2011; Eracleous and 7. et al. 2012; Ju et al. 2013; Liu et al. 2014), the periodical variations (e.g., Valtonen et al. 2008; Graham et al. 2015), etc.; however, most of those candidates are still difficult to confirm. 2. MULTI-BAND OBSERVATIONS OF MRK 231 Thus, it is of great importance to find other ways to select and Mrk 231 is an ultraluminous infrared galaxy with a bright identify BBHs in quasars. Recently, Gültekin & Miller (2012) quasar-like nucleus. It is probably at the final stage of a merger proposed that the continuum emission from a BBH-disk of two galaxies as suggested by its disturbed morphology and accretion system, with unique observable signatures between the associated tidal features (Armus et al. 1994; Lipari et al. 2000 Å and 2 μm because of a gap or a hole in the inner part, 1994). The broadband spectrum of the Mrk 231 nucleus can be used to diagnose BBHs (see Sesana et al. 2012; Roedig exhibits some extreme and surprising properties as follows. et al. 2014; Yan et al. 2014, but Farris et al. 2015). This method First, the flux spectrum (Fl) drops dramatically by a factor of may be efficient in identifying BBHs since many AGNs and ∼10 at the near-UV band (from wavelength l ~ 4000 to quasars have multi-wavelength observations and broadband 2500 Å), while it is flat at λ ∼ 1000–2500 Å and at spectra. Those previous investigations only focus on theoretical λ ∼ 4000–10000 Å. If this sharp drop off is due to extinction,
1 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. of the BBH–disk accretion, e.g., the wind features are consistent with the Fe absorption features of a typical FeLoBAL, and Mrk 231ʼs intrinsic X-ray weakness is also a natural consequence of a BBH–disk accretion system with a small mass ratio.
3. OPTICAL-TO-UV CONTINUUM FROM A BINARY BLACK HOLE—(TRIPLE-)DISK ACCRETION SYSTEM Considering a BBH system resulting from a gas rich merger, the BBH is probably surrounded by a circumbinary disk, and each of the two SMBHs is associated with a mini-disk (see Figure 1). In between the circumbinary disk and the inner mini- Figure 1. Schematic diagram for a BBH–disk accretion system. The BBH is disks, a gap (or hole) is opened by the secondary SMBH, which assumed to be on circular orbits with a semimajor axis of aBBH, and the masses is probably the most distinct feature of a BBH–disk accretion of the primary and secondary components are M·,p and M·,s, respectively. The BBH is surrounded by a circumbinary disk, connecting with the mini-disk system, in analogy to a system in which a gap or hole is opened around each component of the BBH by streams. In between the circumbinary by a planet migrating in the planetary disk around a star (Lin disk and the inner mini-disks, a gap or hole is opened by the secondary SMBH et al. 1996; Quanz et al. 2013). This type of geometric (Artymowicz & Lubow 1996;D’Orazio et al. 2013; Farris et al. 2014). configurations for the BBH–disk accretion systems has been The width of the gap (or hole) is roughly determined by the Hill radius 13 13 revealed by many numerical simulations and analysis (Arty- RH [~aMMBBH()·· ,s30.69 ,p qaBBH], where q is the mass ratio, and the inner boundary of the circumbinary disk can be approximated as mowicz & Lubow 1996; Escala et al. 2005; Hayasaki et al. rin,c~++aqR BBH()1 H. The outer boundary of the mini-disk surrounding 2008; Cuadra et al. 2009;D’Orazio et al. 2013; Farris et al. (r ) ( f ) 4 the secondary SMBH out,s is assumed to be a fraction r,s of the mean Roche 2014; Roedig et al. 2014). The continuum emission from disk 23 23 12 radius, RRL()qaqq 0.49 BBH [ 0.6++ ln ( 1 q )](Eggleton 1983), fi accretion onto a BBH may be much more complicated than that i.e., rout,s = fRr,s RL ( q), considering that the mini-disk may not ll the whole Roche lobe (the red dashed circle). For BBHs with mass ratios roughly in the from disk accretion onto a single SMBH, since the dynamical range from a few percent to 0.25, the accretion onto the secondary SMBH and interaction between the BBH and the accretion flow onto it consequently its emission dominates, compared with that from the mini-disk ( ( ) changes the disk structure Gültekin & Miller 2012; Sesana around the primary BH Roedig et al. 2012; Farris et al. 2014 . et al. 2012;Rafikov 2013; Roedig et al. 2014; Yan et al. 2014; Farris et al. 2015). Nevertheless, we adopt a simple model to – it requires a large dust reddening of Av ~ 7 mag at approximate the continuum emission from a BBH disk l ~-2500 4000 Å and a small dust reddening ∼0.5 mag at accretion system as the combination of the emissions from an <2500 Å (Veilleux et al. 2013). Thus, an unusual complex outer circumbinary disk and an inner mini-disk around the geometric structure for the extinction material surrounding the secondary SMBH, each approximated by multicolor blackbody disk must be designed (Veilleux et al. 2013; Leighly radiation in the standard thin disk model (Novikov & Thorne et al. 2014). 1973; Shakura & Sunyaev 1973). The emission from the mini- Second, Mrk 231 is an extremely powerful Fe II emitter as disk around the primary SMBH is insignificant for a BBH suggested by the spectrum on the blue side of Ha and on both system with a small mass ratio (roughly in the range of a few sides of Hb. Such a high level of optical Fe line emission percent to 0.25) due to its low accretion rate as suggested by suggests a significant amount of Fe line emission at the UV the state of the art numerical simulations (Roedig et al. 2012; band; however, it is not visible in the observed UV spectrum Farris et al. 2014), thus its emission can be neglected. Our ( ) Veilleux et al. 2013; Leighly et al. 2014 . analysis suggests that a large q cannot lead to a good fit to the Third, a number of broad low-ionization absorption line observations. systems, such as Na I D, Ca II,MgII,MgI, and Fe II, have been identified in the optical-to-UV spectrum, which suggests that Mrk 231 should be an Fe low-ionization broad absorption line 3.1. Emission from the Circumbinary Disk quasar (FeLoBAL; Veilleux et al. 2013). However, the expected corresponding absorption features at the UV and We choose a standard thin disk to approximate the FUV bands are not evident. temperature profile of the circumbinary disk. The structure Fourth, the hard X-ray emission of Mrk 231 is extremely and spectral energy distribution (SED) of the circumbinary disk weak. The intrinsic, absorption corrected, X-ray luminosity at may be different from that of a standard thin disk, especially at 42- 1 2–10 keV is L210keV– ~´3.8 10 erg s , and the ratio of the region close to the inner edge, since the torque raised by the ( L210keV– to the bolometric luminosity inferred from the optical central BBH may lead to gas accumulation there. In this region, luminosity) is only ∼0.0003 (Saez et al. 2012; Teng the circumbinary disk is somewhat hotter than the correspond- ) et al. 2014 , almost two orders of magnitude smaller than the ing region of a standard thin disk with the same accretion rate typical value (∼0.03) of a quasar with a similar bolometric and total SMBH mass (MM··,p+ ,s)(e.g., Rafikov 2013).We luminosity (Hopkins et al. 2007). fi neglect this slight difference for now and will discuss this in the Below we show that the above rst feature, the anomalous Appendix. UV continuum, is a distinct prediction of a BBH–disk accretion system as shown in Figure 1. The last three features, commonly 4 ( ) fl The width of the gap or hole is roughly determined by, but could be interpreted under the context of complicated out ows with somewhat larger than, the Hill radius RH. However, set a slightly large gap size, absorptions, can be also accommodated under the framework e.g., 1.2RH, does not affect the results presented in this paper significantly.
2 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. 3.2. Emission from the Inner Mini-disk Associated with the Secondary SMBH We assume that the emission from the inner mini-disks is dominated by that from the mini-disk around the secondary SMBH, and the emission from a mini-disk can also be approximated by that from a standard thin disk with the same extent, accretion rate, and SMBH mass. The accretion onto each of the two SMBHs may be highly variable (Hayasaki et al. 2008; Roedig et al. 2012), and the temperature structure of each of the two mini-disks may be affected by the torque from the ( other SMBH component, the outer circumbinary disk Farris Figure 2. Optical-to-UV spectrum of Mrk 231. The green curve represents the et al. 2015), and the infalling stream. We ignore those UV observations by the COS and FOS on board the HST (archive data; Smith complications in the fitting and will discuss the related effects et al. 1995; Veilleux et al. 2013), the observations by the William Herschel in the Appendix. Telescope (WHT) and Keck telescope (Leighly et al. 2014). The wavelength ranges for those different observations are marked at the bottom of the figure. In the standard thin accretion disk model, the emission from The FOS data is scaled up by a factor of 2 to match the COS data (see Veilleux an annulus r -+dr2 r dr 2 of the disk is approximated et al. 2013). The grey curve represents the HST composite spectrum of quasars by a blackbody radiation with an effective temperature of (Zheng et al. 1997). The continuum shows a sharp drop off at l ~-4000 2500 Å and deficit of flux at l ~-4000 1000 Å. The locations of the emission lines 14 ⎡ 3GM M˙ ⎛ r ⎞⎤ Hα,Hβ, and Lyα are also marked in the figure. Tr()=-⎢ · acc ⎜1,1in ⎟⎥ () eff 3 ⎝ ⎠ ⎣ 8psBr r ⎦ 2 2 FWHMFe =+ FWHMIZw1 FWHMconv . In the above algo- where G is the gravitation constant, sB is the Stefan–Boltzmann ˙ rithm, we assume that the Fe emissions in the UV and optical constant, Macc is the accretion rate of the SMBH, and rin is the bands have the same width and there is no shift between the ’ radius of the disk s inner edge. For the circumbinary disk, redshifts of the UV and the optical lines. We also assume that rin,c=++aqR BBH ()1 H; for the mini-disk around the the ratio of the UV Fe flux to the optical Fe flux is fixed to be 2 secondary SMBH, rin,c~=3.5GM· ,s c 3.5 rg,s, assuming a the same as that of I Zw 1. A detailed dynamical model for the radiative efficiency 0.1 (correspondingly an SMBH spin of broadening and the shift of individual Fe emission lines is 0.67;Yu&Lu2008; Shankar et al. 2013). Here c is the speed beyond the scope of our paper. of light. We can then obtain the continuum emission for either We have also further checked the validity of adopting the Fe ( ) the circumbinary disk or the mini-disk around the secondary template of I Zw 1 by using a CLOUDY Ferland et al. 2013 model, with the best fit continuum of Mrk 231 obtained below SMBH as as the input intrinsic continuum. We find that the total flux of 25 rout 2coshc i l the UV Fe emission (IFe,UV) to the total flux of the optical Fe Fl = dr,2() ò emission (IFe,opt) generated from the CLOUDY model (∼3) is rin exphcl k T() r - 1 []Beff similar to that of I Zw 1 if the ionization parameter is fi (∼ ) where h is the Planck constant, kB the Boltzmann constant, i the suf ciently small 0.001 . A small ionization parameter is 5 compatible with the best-fit parameters of the BBH–disk inclination angle, and rout = 10 rin,c and fRr,s RL () qfor the circumbinary disk and the mini-disk, respectively. Here we accretion system, since the broad line emission region is bound to the primary SMBH, much more massive than the secondary simply assume cosi = 0.8, the mean value for type 1 quasars. fi SMBH, while the ionizing photons are provided by the mini- We nd that a slight change of cos i does not affect our results I I fi disk around the secondary SMBH. If Fe,UV Fe,opt is substan- signi cantly. tially smaller than that of I Zw 1, then the FeLoBAL feature in Mrk 231 would be much less significant than that shown in 3.3. Pseudo-continuum from Fe Emission Lines Figures 3 and 7. It has been shown that there are extremely strong Fe II emission lines in the optical spectrum of Mrk 231. Thousands 4. THE OPTICAL-TO-UV SPECTRUM OF MRK 231 of Fe emission lines from the broad line emission region blending together can form a pseudo-continuum. Therefore, the The observed broadband spectrum of Mrk 231 is first shifted Fe emission must be included when fitting the SED of to the rest frame and then corrected for the Galactic extinction Mrk 231. We use the template-fitting method introduced by [E (–)B V= 0.02213], and the results are shown in Figure 2.A Phillips (1977), probably a standard practice, to treat the Fe number of fitting windows are chosen in order to avoid those emission in Mrk 231, in which the Fe spectrum of the narrow- strong emission or absorption lines, such as Ha, Hb,OIII,Lyα, line Seyfert 1 galaxy I Zw 1 is used to construct an Fe II and Na I D, which is the commonly adopted way to perform the template. In the UV band, we adopt the Fe template of continuum or SED fitting. The fitting windows adopted are Vestergaard & Wilkes (2001). In the optical band, the Fe 1140–1152, 1260–1300, 1340–1400, 1500–1650, 1860–2000, template is constructed according to the list of the Fe lines for I 3090–3145, 3740–3800, 4400–4700, 5080–5600, 6060–6350, Zw 1 given in Véron-Cetty et al. (2004). We combine the UV and 6750–6850 Å. A more complicated model may be and optical Fe templates to form a single template, convolve it developed by fitting each of the emission lines with one or by a Gaussian function with a width of FWHM conv, and scale it more Gaussian components and each of the absorption lines by a factor of As to match the observations. The total flux of the with Voigt profile or Gauss–Hermit expansions; however, these Fe emission, IFe, is equal to As multiplied by the total flux of lines will not affect the overall shape of the continuum, the the template; the FWHM of the Fe lines can be expressed as focus of this paper, and thus we choose to simplify the
3 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. presentation by not involving fittings to those emission and absorption lines. The wavelength range 3800–4400 Å is also excluded because of the contamination from the high-order Balmer lines. The wavelength range from 2000 to 3000 Å is excluded to avoid the uncertainties in modeling strong Fe absorption lines as indicated by the FeLoBAL signatures found in the optical band.
5. MARKOV CHAIN MONTE CARLO (MCMC) FITTING RESULTS We use the MCMC method to obtain the best fittothe observational data in the above fitting windows and constrain the model parameters. The parameters included in the continuum model are the total mass (M·),massratio(q), semimajor axis (aBBH) of the BBH, the Eddington ratios of the outer ( ) ( ) circumbinary disk fEdd,c and the inner mini-disk fEdd,s ,the ratio of the outer boundary of the inner mini-disk to the mean ( ) Roche radius fr,s , the scale factor As and convolution width FWHM conv of the Fe emission lines, and the extinction EBV– due to the interstellar medium in Mrk 231. Here the Eddington ratio is defined as the ratio of the accretion rate of the Figure 3. Optical-to-UV spectrum of Mrk 231 and the model spectrum. From ˙ ( ˙ ) top to bottom, the first panel: the green curve represents the observations as that circumbinary disk Macc,c or the secondary mini-disk Macc,s to in Figure 2. The red curve represents the best-fit model for the continuum the accretion rate M˙ Edd set by the Eddington limit (assuming emission from the BBH–disk accretion system, a combination of the continuum 2 ( ) = 0.1), i.e., fMMMMcLM==˙˙acc,c Edd()·· ˙ acc,c Edd (), emissions from the circumbinary disk cyan dashed line and the mini- Edd,c ( ) ˙˙ ˙ 2 disk around the secondary SMBH cyan dotted line . The second panel: fMMMMcLMEdd,s ==acc,s Edd()·· ,s acc,s Edd() ,s , LEdd ()M· = the green curve represents the observational spectrum of Mrk 231. The red 46- 1 8 1.3´ 10 erg s()MM· 10 ,andLEdd()M· ,s =´1.3 curve represents the best-fit continuum spectrum, a combination of the 46- 1 8 continuum emissions from the circumbinary disk, the mini-disk around the 10 erg s()MM·,s 10 .Wefirst adopt those nine parameters ( ) fi secondary SMBH, and the pseudo continuum by Fe emissions. The third panel: M·, q, aBBH, fEdd,c, fEdd,s, fr,s, As, FWHMconv, EBV– to tthe fi fi fi the green curve represents the residuals of the best t continuum to the spectrum and obtain the best t, and then x the two parameters observations. The blue dotted curve represents the Fe absorption obtained from -1 As and FWHMconv(~ 3000 km s ) at their best-fitvaluesand a model using SYNOW code, and the magenta solid curve represents the + obtain constraints on the other seven parameters (M·, q, aBBH, FeLoBAL absorption of SDSS 1125 0029 scaled by a factor of 1.6. ) fi The fourth panel: the red curve represents the model spectrum by adding the fEdd,c, fEdd,s, fr,s, EBV– .Inthe tting, we adopt an extinction ( ) 5 pseudo-continuum due to numerous Fe emission lines and including curve for SMC according to Pei 1992 ; the Eddington ratios for the contribution from the FeLoBAL absorption. The black points represent the outer circumbinary disk and the inner mini-disk are assumed the observational data in those windows adopted in the continuum fitting as to be in the range from 0.1 to 1, since the standard thin disk marked in this panel. model may be invalid if the Eddington ratio is substantially smaller than 0.1. The top panel of Figure 3 shows the best-fit model to the The second panel of Figure 3 also shows the best fit to the continuum emission from the BBH-disk accretion system. The continuum emission of Mrk 231, which includes not only the overall shape of the observed continuum can be reproduced by continuum emission from the circumbinary disk and the mini- the BBH-disk accretion model. The best fit parameters are disk around the secondary SMBH, but also the pseudo 8 = MM· =´1.5 10 , q 0.026, fEdd,c = 0.5, fEdd,s = 0.6, continuum from numerous Fe emission lines. The Fe emission 6 aBBH = 590 AU2.9= mpc, and fr,s = 0.11, respectively. of Mrk 231 in the optical band is extremely high, and According to the best-fit model, the circumbinary disk consequently the Fe emission in the UV band is likely to be dominates the emission of Mrk 231 in the optical band; the high, too. In the observed UV spectrum, however, there appear mini-disk dominates the FUV emission; the sharp drop off at no strong Fe emission lines (Veilleux et al. 2013; Leighly – Å 4000 2500 is mainly due to the cut off of the circumbinary et al. 2014). Mrk 231 is an FeLoBAL and should have very ( ) disk and the gap or hole opened by the secondary SMBH, but strong Fe absorptions in the UV band as indicated by the not an extremely high extinction (see the top panel of Figure 3). optical absorption lines, such as Na I D, but there appear no It is also not a necessity to have different extinctions in very strong Fe absorption lines in the observed UV spectrum. different bands to explain the sharp drop off. The reason is that the Fe absorption dominates over the Fe emission in this band as detailed below. 5 We have checked that choosing a different extinction curve (e.g., the one for LMC) does not qualitatively affect our results. The third panel of Figure 3 shows the residuals of the model 6 fi – Å Note that the best t value of fEdd,s = 0.6 means that the secondary SMBH spectrum at 2000 3200 which reveals strong Fe absorption accretes via a rate close to the Eddington limit. The numerical simulations lines. These Fe absorption features are quite similar to a 1.6- suggest that the accretion rate onto the primary SMBH is smaller than that onto the secondary SMBH for a BBH-disk accretion system with a mass ratio of a timescale up of the FeLoBAL absorption features of SDSS 1125 few to 25% (e.g., Farris et al. 2014). For the BBH system that best fits the +0029 shown in Figure 4(a) of Hall et al. (2002, the magenta continuum, the accretion rate of the primary disk should be 0.01, which is via curve), of which no observation at 2000 Å is available. As an the advection dominated accretion flow (ADAF) mode (Esin et al. 1997) and ( radiatively extremely inefficient, and thus its emission can be neglected. This illustration, we further use the SYNOW code Branch validates the omission of the primary disk emission in the fitting. et al. 2002), a fast parameterized synthetic-spectrum code, to
4 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al.
Figure 4. Two-dimensional probability contours for different parameters of the BBH. The red stars represent the best-fit parameter values. Only 50,000 points (black) out of 50,000,000 simulations are plotted. The cyan curves represent 1, 2, and 3σ confidence contours from the inside to the outside, respectively.
Figure 5. One-dimensional probability distributions of different parameters of the BBH. The red dotted vertical line in each panel represents the value of the best fit adopted in Figures 2 and 3 to produce the model spectrum. generate a spectrum for the absorption features in the NUV band curve) is a combination of the best fit to the continuum and a for Mrk 231. We assume that only the wind that covers the 1.6-timescale up of the FeLoBAL absorption feature of SDSS surface of the disk facing toward distant observers contributes to 1125+0029 (cyan line in Figure 3), which appears to match the the absorption because of the optically thick disk, and this wind observations well. may launch at the inner edge of the circumbinary disk and/or Figures 4 and 5 show the two-dimensional probability from the mini-disk. For simplicity, we consider three species, contours and one-dimensional marginalized probability distribu- i.e., Fe I,FeII,andMgII, and assume a temperature of 10000 K, tions for the model parameters obtained from the MCMC fitting, -1 minimum velocity vmin = 1000 km s , and maximum velocity respectively. The parameters M·, fEdd,andaBBH mainly -1 vmax = 8000 km s . The maximum velocity is on the same determine the emission from the circumbinary disk and they order of the escape velocity at aBBH. The model absorption are strongly degenerate with each other. If one of them could be spectrum is shown in the bottom panel of Figure 3.Itappears determined by an independent method, the constraints on the that the main absorption features seen in the residuals can be other two would be significantly improved. The emission from roughly modeled, though there are still some discrepancies in the mini-disk around the secondary SMBH is determined by q, ( details. In the bottom panel of Figure 3, the model spectrum red fEdd,s,and fr,s,forwhichq and fEdd,s are also degenerate with 5 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. each other. The constraints on these three parameters can be substantially improved if observations at EUV (e.g., 1000 Å) are available. According to the marginalized one-dimensional probability distribution for each parameter shown in Figure 5 by the standard MCMC technique, we have the peak values of the +0.2 +0.2 model parameters as log()MM· = 8.3-0.2, logq =- 1.8-0.2, +0.4 +0.3 logfEdd,c =- 0.5-0.3, logfEdd,s =- 0.4-0.3, log()aRBBH g = +0.2 ( 2) +0.4 2.5-0.2 here Rg = GM· c , logfr,s =- 0.5-0.4, EBV– = +0.03 +0.2 0.10-0.03, and the orbital period 1.2-0.1 years. Note these values for the model parameters are somewhat different from those from the best fit with minimum c2-value. The X-ray emission of AGNs and quasars is normally emitted from a corona structure above the inner disk area in the Figure 6. UV continuum flux at 1300 Å (L1300) for Mrk 231 at different standard disk–corona model (Haardt & Maraschi 1993; Dai epochs. Different points are obtained from the UV spectra obtained by IUE and et al. 2010). Here in Mrk 231, the outer circumbinary disk is HST at different epochs as marked in the figure, smoothed over a wavelength truncated at a radius substantially larger than several hundreds range of 20 Å around 1300 Å at the rest frame. The bars associated with each of the Schwarzschild radius of the primary SMBH, for which point represent the 1σ standard deviation of the measurement. the disk is too cold and a corona probably cannot be established fi above it to emit X-rays signi cantly. Therefore, the X-ray The inclination angle of the disk in Mrk 231 could be larger, emission can only arise from the mini-disk or its associated ( – e.g., cosi ~ 0.5, since Mrk 231 is an FeLoBAL though not a corona around the secondary SMBH in the BBH disk accretion normal FeLoBAL in the BBH–disk accretion scenario) and model. Since the secondary SMBH is smaller than the primary ∼ / ∼ FeLoBALs were suggested to have a larger i and a smaller cos i one by a factor of 1 q 38, it is almost guaranteed that the (Goodrich & Miller 2005). By alternatively setting cosi = 0.5, X-ray emission from this system is much weaker than that from fi fi – the best t obtained from the MCMC tting suggests a single SMBH disk accretion system with an SMBH mass of log()MM = 8.5+0.2, logq =- 1.9+0.2, logf =- 0.5+0.3, MM+ and an accretion rate of fM˙ (or · -0.2 -0.2 Edd,c -0.4 ··,p ,s Edd,c · Edd,c logf =- 0.3+0.2, log()aR= 2.5+0.2, log f = fM˙ ). The best fit suggests that the observed X-ray Edd,s -0.2 BBH g -0.2 r,s Edd,s · Edd,c +0.3 +0.02 fi luminosity of Mrk 231 in the 2–10 keV band is about 1% of the -0.5-0.3, and EBV– = 0.07-0.02. With this BBH con guration, +0.3 bolometric luminosity from the mini-disk, which is well the orbital period is 1.6-0.2 years and W 40.5p . It appears consistent with those for normal AGNs and quasars (Hopkins that the results are qualitatively consistent with those obtained et al. 2007). This solves the mystery of the intrinsic X-ray by assuming cosi = 0.8. By relaxing the inclination angle as a weakness of Mrk 231. free parameter, we find that our results are not affected significantly. Numerical simulations suggest that the continuum emission 6. MODEL IMPLICATION AND DISCUSSIONS from a BBH–disk accretion system may vary periodically due to the change of the material infalling rate from the fi According to the model ts, the intrinsic continuum and circumbinary disk to the inner mini-disk(s)(e.g., Hayasaki fi SED of Mrk 231 is signi cantly different from the canonical et al. 2008), though the variation for those systems with small fi ones of normal quasars. The de cit of intrinsic UV emission mass ratios (less than a few percent) may not be significant will lead to substantially weaker broad line emissions (e.g., Farris et al. 2014). Several UV observations of Mrk 231 compared with those of normal quasars. However, a number have been obtained by Hubble Space Telescope (HST) over the of broad emission lines, such as Ha, Hb, are evident in the years. The UV continuum of the HST COS observations by α fl optical spectrum of Mrk 231. The ratio of the H ux to Veilleux et al. (2013) is roughly consistent with the HST β fl the H ux is roughly 3, consistent with those of normal G160H observation by Gallagher et al. (2002), while it is quasars. Here we check whether the total number of ionizing perhaps a factor of two higher than the HST G190/270 H photons emitted by the central source is large enough to observation by Smith et al. (1995). As seen from Figure 6, balance the total number of recombinations occurred in the earlier observations by IUE (Hutchings & Neff 1987) show that broad line region. The total number of H photons is directly b the continuum (lLl) levels of Mrk 231 at 1300 Å ~ 2.30, 3.20, L = ---12 2 1 related to the number of ionizing photons as Hb and 3.21´ 10 erg cm s at three different epochs, and eff 0 ---12 2 1 W aH ()H,T ¥ later on they are about 1.85 and 1.77´ 10 erg cm s b hdnnLn , where W 4p is the covering 0 Hb ò hn measured by the HST COS (Veilleux et al. 2013) and FOS 4p a ()H,T n0 B (Gallagher et al. 2002). The probability for those observations factor, aaeff ~ 18.5is the number of H photons produced Hb B b to be consistent with a constant (or no variation) is 0.00013 Å 2 per hydrogen recombination, and n0 = c 912 , nHb = c 4861 according to the c -statistics, which suggests that the UV Å (Osterbrock et al. 2006). Subtracting the best-fit continuum emission over the past several decades does vary, confirming from the observed spectrum from 4700 Å to 5800 Å, we obtain the claim by Veilleux et al. (2013). However, these observa- Hb luminosity by integrating the residual spectrum. Using the tions are not sufficient for investigating the expected (quasi-) above equation, we find W~40.29p , which is fully periodical variations as done for the recently reported BBH consistent with the typical range of Ω for AGNs/quasars and candidate PG 1302-102 (Graham et al. 2015) because (1) the suggests that the ionizing photon emission from the mini-disk total number of the observations is small; and (2) the intervals around the secondary SMBH is sufficient to produce the optical between some of these observations are too large compared broad emission lines, such as Hα and Hβ. with the expected orbital period. Although there are more
6 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. optical observations of Mrk 231, the number of observations at The spectral features of Mrk 231 were recently proposed to each individual band is limited and not sufficient for an be explained by either (1) a hybrid model with both a young analysis on the (quasi-)periodical variation. star burst (for the UV continuum) and an obscured quasar (for Note that the periodical variation of the BBH candidate the optical continuum; see Leighly et al. 2014) or (2) an PG 1302-102 (Graham et al. 2015) has an amplitude of only absorption model with extremely large dust reddening at the ∼0.14 magnitude (or ∼14%), which suggests that the NUV band but small reddening at the FUV band (see Veilleux amplitude of the expected periodical variations of some et al. 2013). In the first model, the FUV emission is dominated BBH-disk accretion systems may be small. Future intensive by the young star burst with an age of ∼100 Myr and therefore monitoring the UV continuum emission of Mrk 231 may reveal would have no significant short time-scale variations, which the periodical variation, which would confirm the BBH appears inconsistent with the observations. Veilleux et al. ( ) α hypothesis and be useful to further constrain the dynamical 2013 also argued that the broad asymmetric Ly line cannot interplay between the BBH and its surrounding accretion flow. be produced if the FUV emission is from an extended star It has been shown that the polarization fraction of the burst. While in the second model, a small fraction of FUV optical-to-UV continuum of Mrk 231 depends on frequency emission is leaked through the obscuring material, and the UV and the peak of the polarization is around the wavelengths variability will depend on the stability of the leaking holes. Future monitoring observations of Mrk 231 at the FUV and ∼3000 Å (Smith et al. 1995). This dependence is probably due NUV bands, and simultaneous multi-wavelength observations to the scattering clouds distributed asymmetrically about the will be helpful to confirm the BBH–disk accretion explanation illuminating source (Smith et al. 1995). In the BBH scenario, Å and further constrain the orbital evolution of the BBH in the the blue photons around 3000 are mainly emitted from the core of Mrk 231. inner edge of the circumbinary disk, where the disk may be puffed up because of the accumulation of accreting hot material there, which may lead to more significant scattering of those 7. CONCLUSIONS blue photons emitted from that region and thus a high In this paper, we show that various unique features in the polarization fraction; while those photons at the FUV and optical-to-UV spectrum and the intrinsic X-ray weakness of optical bands may experience less scattering as they are away Mrk 231 can all be well explained, if a pair of SMBHs exists in from the inner edge of the circumbinary disk. This may explain the core of Mrk 231, with the masses of the primary and 8 6 the frequency dependence of the polarization fraction, the secondary SMBHs as ~´1.5 10 M and 4.5´ 10 M, qualitatively; Veilleux et al. (2013) and Leighly et al. (2014), respectively. The existence of a BBH in Mrk 231 is compatible likewise, also provided qualitative explanations on these with its disturbed morphology and tidal features, which polarization measurements in their competing models. indicates a merger event in the past. (Note that the secondary The standard accretion disk model adopted in the fitting is SMBH is rather low in mass; however, it should be able to sink simple, and the torque due to the BBH on the outer disk is not down to the center because the stars initially associated with it considered. As shown in the Appendix, we adopt the model by enhance the dynamical friction see Yu 2002.) The semimajor Rafikov (2013) to fit the continuum, in which both the internal axis of this BBH is ∼590 AU, about 190 times of the viscosity and the external torque by the BBH on the Schwarzschild radius of the primary SMBH, and its orbital circumbinary disk are considered, and we find that there are period is just ∼1.2 years, relatively short among the few known no significant differences in the constraints on the model BBH candidates (Valtonen et al. 2008; Graham et al. 2015), parameters. which makes it an ideal system to study the dynamics of BBH According to the best fit, the infalling rate from the systems. Such a BBH emits gravitational wave on tens of circumbinary disk to the inner disk(s) is smaller than the nanohertz, and the change rate of its orbital period due to accretion rate of the circumbinary disk by a factor of ∼30. gravitational wave radiation is about 40 s per orbit. This BBH Currently it is not clear whether such a small infalling rate can might be a target for gravitational wave studies in future. be realized in BBH–disk accretion systems. For close BBH– The orbit of such a BBH system decays on a timescale of a few times of 105 years due to gravitational wave radiation and disk accretion systems, a number of simulations and analyses ( ) suggested that the infalling rate into the gap or the central the torque of the circumbinary disk Haiman et al. 2009 , which is not too small compared with the lifetime of quasars (a few cavity is substantially smaller than the accretion rate at the times 107 to 108 years; see Yu & Tremaine 2002;Yu& outer boundary of the circumbinary disk because of the tidal Lu 2008; Shankar et al. 2013). The majority of quasars are barrier by the central BBH (e.g., the 1D simulations by ć believed to be triggered by mergers of galaxies and Milosavljevi & Phinney 2005, the 3D simulations by consequently involve mergers of SMBHs (Volonteri Hayasaki et al. 2007, or the simple arguments by Rafikov ) ) et al. 2003 , and those BBH systems with mass ratio in the 2013 . However, some recent simulations showed that the range of a few percent to 1 may lead to a notch in the optical- fi infalling rate may not be signi cantly suppressed by the tidal to-UV continuum emission if their semimajor axes are in the ( ć barrier e.g., MacFadyen & Milosavljevi 2008; Noble et al. range of a few hundreds to about one thousands gravitational 2012) and it may be even comparable to the accretion rate in radii (Roedig et al. 2014), which correspond to orbital decay the case with a single central MBH (Farris et al. 2014; Shi & timescales of 105–106 years (Haiman et al. 2009). Therefore, Krolik 2015). Considering that all those simulations are quite the occurrence rate of active BBH systems, with deficits in the idealized, it is also not clear whether the accretion onto the optical-to-UV emission, may be roughly a few thousandths to central BBH is really significantly suppressed or not. Future about one percent among quasars (Yan et al. 2014). Our more realistic simulations may help to demonstrate whether the analysis of Mrk 231 demonstrates the feasibility of finding small infalling rate found for the BBH-disk accretion system in BBH systems by searching for the deficits in the optical-to-UV Mrk 231 is possible. emission among the spectra of quasars, a new method proposed
7 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. by a number of authors (Gültekin & Miller 2012; Sesana et al. 2012; Yan et al. 2014; Farris et al. 2015), in addition to those current practices by searching for the kinematic and image signatures of BBHs among AGNs/quasars (Gaskell 1996; Rodriguez et al. 2006; Bogdanovic et al. 2008; Valtonen et al. 2008; Boroson & Lauer 2009; Tsalmantza et al. 2011; Eracleous et al. 2012; Popović 2012; Ju et al. 2013; Liu et al. 2014; Graham et al. 2015).
This work was supported in part by the National Natural Science Foundation of China under grant nos. 11273004 (Q.Y.), 11103029 (C.Y.), 11373031 and 11390372 (Y.L.), and the Strategic Priority Research Program “The Emergence of Cosmological Structures” of the Chinese Academy of Sciences, Grant No. XDB09000000 (Y.L.). X.D. is supported by the NSF grant AST-1413056.
APPENDIX Figure 7. Optical-to-UV spectrum of Mrk 231 and the model spectrum. Legend is similar to that for Figure 3, except that the torque at the inner edge of the circumbinary disk is considered in the model fitting to the continuum The structure of the circumbinary disk may be different from emission. that of a standard thin disk, especially at the region close to the inner edge, since the torque raised by the central binary may lead to gas accumulation there. According to Rafikov (2013), may gradually accumulate near the inner boundary if c < 1. ˙ the evolution of the circumbinary disk under the actions of both We set the accretion rate at the outer boundary as M¥. A self- the internal viscosity and the external torque can be described similar solution can be obtained for the evolution of the by a simple equation circumbinary disk once the initial separation of the BBH is [ ] ⎡ ⎤ given i.e., aBBH()ta==0 BBH, 0 . With the solution of ¶S 12¶ ⎛ ¶l ⎞-1 ¶ ⎛ ¶W ⎞ SL =- ⎢ ⎜⎟ ⎜r 3nS ⎟+ ⎥.3() FrtJ (), , the effective temperature of the disk can be obtained as ¶trr¶ ⎣ ⎝ ¶rr⎠ ¶ ⎝ ¶r ⎠ W ⎦ ⎡ 3 Frt(), W ⎤14 Trt(), = ⎢ J ⎥ .6() Here t is the evolution time, Λ is the external torque per unit eff ⎣ 2 ⎦ 8psBr mass of the disk due to the central BBH, Σ, ν, and r are the surface density, kinematic viscosity, and radius of the disk, By integrating the multi-color blackbody emission over the respectively, and lrr=W()2 is the specific angular momentum whole circumbinary disk, we obtain the continuum flux as of gas material on a circular orbit with a radius r. At each 25 rout,c 2coshc i l radius, the accretion disk properties can be characterized by the Fl,c = ò dr.7() viscous angular momentum flux rin,c exp[]hcl kBeff T() r - 1 Here h is the Planck constant, k is the Boltzmann constant, 32dW B FrJ º-23,4pn S =SW pn r () 5 2 dr rin,c=++at BBH () (1 qR ) H, rout,c is set to be 10 GM· c , and aBBH ()t is the semimajor axis of the BBH at time t. The where the last equality is valid for a near Keplerian disk with evolution of a ()t is controlled by 31 2 BBH W ()GM· r and MM··=+,p M · ,s. Λ ( ) fi da a a Since in Equation 3 is signi cant only in a narrow annulus BBH=- BBH - BBH ,8() at the inner edge of the disk, we can assume that L=0 outside dt tGW tJ r r r of some radius L, which is not too different from in.Outside L, 2 4 the disk evolves according to Equation (3) with L=0.The where tqqRcaRGW =+[()]()()51 64 g BBH g is the grav- 2 solution of this equation can be obtained by the standard itation wave radiation timescale Peters (1964), Rg = GM· c is ( ) approach introduced in Lynden-Bell & Pringle 1974 ,ifthe the gravitational radius, and tLJº BBH 2 FJ is the characteristic inner boundary condition (IBC) isassumedtobetorquefree. timescale for the BBH orbit shrinking due to the coupling to the However, the central BBH exerts a torque on the inner boundary circumbinary disk. If we adopt this model to describe the of the circumbinary disk, which can be approximated by circumbinary disk and adopt the standard thin disk model to FJ () rin»- dL BBH dt =- L BBH v BBH2 a BBH,whereLBBH = describe the mini-disk, we can also fit the continuum of 12 2 qM· ()() GM· aBBH 1 + q is the orbital angular momentum Mrk 231 by the MCMC technique. Under this approach, of the BBH, and vdadtº is the inspiralling speed of BBH BBH ffq= c . The best fit is shown in Figure 7, and the the BBH. According to Rafikov (2013), this IBC may be written Edd,s Edd,c 4 as constraint on aBBH,0 and t are 355Rg and 2.1´ 10 years, respectively. Other best-fit parameters are MM= 108.2 , ¶F · J ˙ ˙ f = 0.4, q = 0.02, c = 0.04 (corresponding to rr=¥in ==Mr()in c M,5() Edd,c ¶l ) ( fEdd,s = 0.7 , aBBH=~343R g 540 AU corresponding to χ ( ) ) where is assumed to be a constant 1 . By this setting, rin,c= 402R g, fr,s = 0.12 , and EBV– = 0.14. These results Mr˙ ()in can be substantially smaller than M˙¥, and disk material are roughly consistent with those shown in Figure 3, which are
8 The Astrophysical Journal, 809:117 (9pp), 2015 August 20 Yan et al. obtained without considering the torque of the BBH on the Gültekin, K., & Miller, J. M. 2012, ApJ, 761, 90 inner boundary of the circumbinary disk. Haardt, F., & Maraschi, L. 1993, ApJ, 413, 507 Haiman, Z., Kocsis, B., & Menou, K. 2009, ApJ, 700, 1952 The torque due to the outer circumbinary disk on the mini- Hall, P. B., Anderson, S. F., Strauss, M. A., et al. 2002, ApJS, 141, 267 disk around the secondary SMBH and the shock induced by the Hayasaki, K., Mineshige, S., & Ho, L. C. A. 2008, ApJ, 682, 1134 infall streams onto the mini-disk (Lodato et al. 2009; Kocsis Hayasaki, K., Mineshige, S., & Sudou, H. 2007, PASJ, 59, 427 et al. 2012; Roedig et al. 2014; Farris et al. 2015), which are Hopkins, P. F., Richards, G. T., & Hernquist, L. 2007, ApJ, 654, 731 omitted in this study, may also introduce some errors to the Hutchings, J. B., & Neff, S. G. 1987, AJ, 93, 14 fi fl Ju, W., Greene, J. E., Rafikov, R. R., Bickerton, S. J., & Badenes, C. 2013, tting. Although many simulations and analysis suggest a ux ApJ, 777, 44 deficit in the continuum emission from a BBH-disk accretion Kocsis, B., Haiman, Z., & Loeb, A. 2012, MNRAS, 427, 2680 system because of the gap (or hole) in the disk, including the Komossa, S., Burwitz, V., Hasinger, G., et al. 2003, ApJL, 582, L15 one by Roedig et al. (2014) with consideration of the shock Kormendy, J., & Ho, L. C. 2013, ARA&A, 51, 511 ( ) Leighly, K. M., Terndrup, D. M., Baron, E., et al. 2014, ApJ, 788, 123 heating hot-spot on the inner mini-disk s , one new simulation Lipari, S., Colina, L., & Macchetto, F. 1994, ApJ, 427, 174 by Farris et al. (2015) suggested that there may be no strong Lin, D. N. C., Bodenheimer, P., & Richardson, D. C. 1996, Natur, 380, 606 flux deficit in the continuum emission of a BBH-disk accretion Liu, X., Greene, J. E., Shen, Y., & Strauss, M. A. 2010, ApJL, 715, L30 system by considering the emission from the infalling streams. Liu, X., Shen, Y., Bian, F., Loeb, A., & Tremaine, S. 2014, ApJ, 789, 140 However, only a specific case with a mass ratio of 1 is Lodato, G., Nayakshin, S., King, A. R., & Pringle, J. E. 2009, MNRAS, ( ) 398, 1392 considered in Farris et al. 2015 and the stream emission Lynden-Bell, D., & Pringle, J. E. 1974, MNRAS, 168, 603 therein is approximated as a thermal emission without MacFadyen, A. I., & Milosavljević, M. 2008, ApJ, 672, 83 considering inverse Compton scattering and radiative transfer. Magorrian, J., Tremaine, S., Richstone, D., et al. 1998, AJ, 115, 2285 Therefore, their results might not be applicable to other cases Merritt, D., & Milosavljević, M. 2005, LRR, 8, 8 ( Milosavljević, M., & Phinney, E. S. 2005, ApJL, 622, L93 and need to be improved see the discussion in Farris Noble, S. C., Mundim, B. C., Nakano, H., et al. 2012, ApJ, 755, 51 et al. 2015). Roedig et al. (2014) argued that the shock induced Novikov, I. D., & Thorne, K. S. 1973, in Black Holes, ed. C. De Witt & by the infalling streams on the mini-disk(s) may enhance the B De Witt (New York: Gordon and Breach), 343 X-ray emission at 100 keV, which does not affect the X-ray Osterbrock, D. E., & Ferland, G. J. 2006, in Astrophysics of Gaseous Nebulae fi and Active Galactic Nuclei, ed. D. E. Osterbrock & G. J. Ferland (2nd ed.; emission with energy signi cantly less than 100 keV and thus ) – Sausalito, CA: Univ. Science Books , 343 does not affect our estimate on the 2 10 keV emission. Pei, Y. C. 1992, ApJ, 395, 130 Peters, P. C. 1964, Phy Rev, 136, 1224 REFERENCES Phillips, M. M. 1977, ApJ, 215, 746 Popović,L.Č. 2012, NewAR, 56, 74 Armus, L., Surace, J. A., Soifer, B. T., et al. 1994, AJ, 108, 76 Quanz, S. P., Avenhaus, H., Buenzli, E., et al. 2013, ApJL, 766, L2 Artymowicz, P., & Lubow, S. H. 1996, ApJL, 467, L77 Rafikov, R. R. 2013, ApJ, 774, 144 Begelman, M. C., Blandford, R. D., & Rees, M. J. 1980, Natur, 287, 307 Rodriguez, C., Taylor, G. B., Zavala, R. T., et al. 2006, ApJ, 646, 49 Bogdanovic, T., Smith, B. D., Sigurdsson, S., & Eracleous, M. 2008, ApJS, Roedig, C., Krolik, J. H., & Miller, M. C. 2014, ApJ, 785, 115 174, 455 Roedig, C., Sesana, A., Dotti, M., et al. 2012, A&A, 545, A127 Boroson, T. A., & Lauer, T. R. 2009, Natur, 458, 53 Saez, C., Brandt, W. N., Gallagher, S. C., Bauer, F. E., & Garmire, G. P. 2012, Branch, D., Leighly, K. M., Thomas, R. C., & Baron, E. 2002, ApJL, 578, L37 ApJ, 759, 42 Comerford, J. M., Pooley, D., Gerke, B. F., & Madejski, G. M. 2011, ApJL, Sesana, A., Roedig, C., Reynolds, M. T., & Dotti, M. 2012, MNRAS, 420, 737, L19 860 Cuadra, J., Armitage, P. J., Alexander, R. D., & Begelman, M. C. 2009, Shakura, N. I., & Sunyaev, R. A. 1973, A&A, 24, 337 MNRAS, 393, 1423 Shankar, F., Weinberg, D. H., & Miralda-Escudé, J. 2013, MNRAS, 428, 421 Dai, X., Kochanek, C. S., Chartas, G., et al. 2010, ApJ, 708, 278 Shi, J.-M., & Krolik, J. H. 2015, ApJ, 807, 131 D’Orazio, D. J., Haiman, Z., & MacFadyen, A. 2013, MNRAS, 436, 2997 Smith, P. S., Schmidt, G. D., Allen, R. G., & Angel, J. R. P. 1995, ApJ, Eggleton, P. P. 1983, ApJ, 268, 368 444, 146 Eracleous, M., Boroson, T. A., Halpern, J. P., & Liu, J. 2012, ApJS, 201, 23 Teng, S. H., Brandt, W. N., Harrison, F. A., et al. 2014, ApJ, 785, 19 Escala, A., Larson, R. B., Coppi, P. S., & Mardones, D. 2005, ApJ, 630, 152 Tsalmantza, P., Decarli, R., Dotti, M., & Hogg, D. W. 2011, ApJ, 738, 20 Esin, A. A., McClintock, J. E., & Narayan, R. 1997, ApJ, 489, 865 Valtonen, M. J., Lehto, H. J., Nilsson, K., et al. 2008, Natur, 452, 851 Farris, B. D., Duffell, P., MacFadyen, A. I., & Haiman, Z. 2014, ApJ, 783, 134 Veilleux, S., Trippe, M., Hamann, F., et al. 2013, ApJ, 764, 15 Farris, B. D., Duffell, P., MacFadyen, A. I., & Haiman, Z. 2015, MNRAS, Véron-Cetty, M.-P., Joly, M., & Véron, P. 2004, A&A, 417, 515 446, L36 Vestergaard, M., & Wilkes, B. J. 2001, ApJS, 134, 1 Ferland, G. J., Porter, R. L., van Hoof, P. A. M., et al. 2013, RMxAA, 49, 137 Volonteri, M., Haardt, F., & Madau, P. 2003, ApJ, 582, 559 Fu, H., Yan, L., Myers, A. D., et al. 2012, ApJ, 745, 67 Yan, C.-S., Lu, Y., Yu, Q., Mao, S., & Wambsganss, J. 2014, ApJ, 784, 100 Gallagher, S. C., Brandt, W. N., Chartas, G., Garmire, G. P., & Yu, Q. 2002, MNRAS, 331, 935 Sambruna, R. M. 2002, ApJ, 569, 655 Yu, Q., & Lu, Y. 2008, ApJ, 689, 732 Gaskell, C. M. 1996, ApJL, 464, L107 Yu, Q., & Tremaine, S. 2002, MNRAS, 335, 965 Goodrich, R. W., & Miller, J. S. 2005, ApJL, 448, L73 Zheng, W., Kriss, G. A., Telfer, R. C., Grimes, J. P., & Davidsen, A. F. 1997, Graham, M. J., Djorgovski, S. G., Stern, D., et al. 2015, Natur, 518, 74 ApJ, 475, 469
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